1. Field of the Invention
The present disclosure relates to a semiconductor device applicable to a power switching element which is used in an inverter or a power supply circuit.
2. Description of the Related Art
Recently, as a power switching element, an FET (field effect transistor) composed of a nitride semiconductor material represented by gallium nitride (GaN) has been enthusiastically studied. More specifically, GaN has a bandgap about three times wider than silicon (Si), and its breakdown field intensity is ten times higher than that of Si. Furthermore, compared with Si, GaN has higher saturation electron speed, so that GaN has excellent material characteristics that a device can be operated at high speed. At present, as a power switching element, a MOSFET (metal oxide semiconductor field effect transistor) and an IGBT (insulated gate bipolar transistor) composed of Si are used, but when Si is replaced with GaN in the FET, it is expected that the element can be made smaller and lighter, and a power loss can be reduced.
In order to reduce the power loss, an ON-resistance needs to be low in the power switching element. In addition, in view of safety purposes, a normally-OFF operation in which a current is cut off in a zero-biased state is indispensable. As a technique realizing both of the normally-OFF operation and the low ON-resistance in the FET composed of GaN, there is an example in which a p-type nitride semiconductor layer is used in a gate portion, and a gate recess is formed under the p-type nitride semiconductor layer (in the first exemplary embodiment of Unexamined Japanese Patent Publication No. 2009-200395). With this structure, a two-dimensional electron gas concentration can be reduced right under a gate electrode, so that both of the normally-OFF operation and the low ON-resistance can be achieved.
Furthermore, as another background technique, there is an example in which a diffusion layer of a p-type impurity is formed under a p-type nitride semiconductor layer in a gate portion (in the second exemplary embodiment in the Unexamined Japanese Patent Publication No. 2009-200395). In this example also, similar to the above example, a two-dimensional electron gas concentration can be reduced right under a gate electrode.